This invention relates to a ceramic composition for a PTC (positive temperature coefficient) resistor and, more specifically to a ceramic composition for a PTC resistor which is characterized by having a small specific resistance in the state of a low resistance.
Heretofore, as typical materials for the PTC resistor, there have been used BaTiO.sub.3 ceramics in which a variety of impurities are included. For example, BaTiO.sub.3 ceramics in which La, Sm, Sb or Nb is included shows PTC properties in that the relative resistance thereof increases about 10.sup.4 times at around 250.degree. C. as compared with those of at ambient temperature (J. Mat. Sci., Vol. 6, p. 1214 (1971); W. Heywang). These ceramics have as large an electric resistance at 10.sup.0 .OMEGA.cm or more in a low resistance condition and their PTC phenomenon depends on a mechanism which is based on grain boundary layers; therefore they can scarcely be utilized in fields utilizing a large electric power.
It is known that the compound V.sub.2 O.sub.3 in which Cr or Al is included has PTC properties of a specific resistance based on the fact that it transfers from a metallic state to an insulating state at from room temperature to about 200.degree. C. For example, a V.sub.2 O.sub.3 single crystal in which Cr is included, shows PTC properties in that the relative resistance thereof increases from 10.sup.-2 .OMEGA.cm to 1 .OMEGA.cm with increasing temperature at around room temperature (Phys. Rev. B7, p. 1920 (1973); D. B. McWhan et al.). In a V.sub.2 O.sub.3 single crystal in which Al is included, the same PTC properties as mentioned above have been observed (Phase Transitions, 1, P. 289 (1980); H. Kuwamoto & J. M. Honig). However, it is hard to prepare these materials in the form of a large single crystal. Further, their polycrystal sinters are poor in sintering characteristics, accordingly high-density ceramics are difficult to obtain from them. Futhermore, the specific resistance of the PTC properties in the low resistance state is about 10 times as high as that of the single crystal; therefore it is hard to obtain a high PTC magnification. In addition, by since being low in strength owing to their low density, such polycrystal sinters cannot be applied to fields utilizing a large electric power.